Method of making high resistivity group iii-v compounds and alloys doped with iron from an iron-arsenide source



R. W. G HIGH CONRAD ET AL 3,421,952

ISTIVITY GROUP 111w COMPOUNDS D WITH N FROM AN IRON-ARSENIDE SOURCEFiled Feb. 2, 1966 Jan. 14, 1969 METHOD OF MA AND ALLOYS n Raymond W.Conrad Robert W. Haisfy Pele L. Hoyi ooooooooooooooooao X/ //YOOOOOOOOOOOOOOOOOOO QQYJM ATTORNEY United States Patent 3,421,952 METHODOF MAKING HIGH RESISTIVITY GROUP III-V COMPOUNDS AND ALLOYS DOPED WITHIRON FROM AN IRON-ARSENIDE SOURCE Raymond W. Conrad, Dallas, Tex.,Robert W. Haisty,

Fellbach, Stuttgart, Germany, and Pete L. Hoyt, Garland, Tex., assignorsto Texas Instruments Incorporated, Dallas, Tex., a corporation ofDelaware Filed Feb. 2, 1966, Ser. No. 524,538 US. Cl. 148-175 Int. Cl.H01l 7/00 This invention relates to high resistivity or semi-insulatingGroup III-V compounds and alloys and methods of making same. Moreparticularly, it relates to a method of producing epitaxial deposits ofhigh resistivity Group III-V compound materials through the vapor phasereaction of Group III and Group V elements while simultaneously dopingwith appropriate impurities to produce epitaxial deposits withresistivities of about ohm-cm. or greater.

As used herein, the terms high resistivity Group III- V materials andsemi-insulating Group IIIV materials are used to refer to compounds ofone or more elements from Group III of the Periodic Table with one ormore elements of Group V of the Periodic Table, said compounds having aresistivity of the order of 10 ohm-cm. or higher. The terms refer toternary and quaternary compounds and alloys of the Group III-V compoundsas well as binary compounds of Group III and Group V elements.

Semi-insulating Group III-V compound crystals have been produced bywithdrawing a crystalline seed from an appropriately doped melt of thesemiconductor material. Pulled crystals of semi-insulating galliumarsenide, for example, may be formed by doping a GaAs melt with oxygen,iron or chromium. In the past, however, epitaxial deposits of uniformlyhigh resistivity Group III-V materials could not be controllablyproduced. Various attempts to dope Group III-V epitaxial deposits withoxygen, iron or chromium during the production thereof to producecontrolled deposits of uniformly high resistivity monocrystalline GroupIII-V materials have been generally unsuccessful. Apparently thepresence of oxygen in an epitaxial reactor in amounts sufficient to dopethe deposit to high resistivity interferes with the formation ofmonocrystalline deposits. Chromium and iron, although known for theirability to form semi-insulating III-V crystals grown from a melt, couldnot be included in sufficient amounts in epitaxial deposits to dope thedeposit to high resistivities.

It is therefore an object of this invention to provide a method ofmaking epitaxial monocrystalline layers of high resistivity Group III-Vmaterials. Another object is to provide epitaxial layers ofsemi-insulating Group III- V materials on semiconductor substrates andto make composite structures of alternate layers of high resistivity andsemiconducting materials. Still another object is to provide a method ofdoping III-V epitaxial deposits with iron in sufiicient quantities toproduce monocrystalline epitaxial deposits with resistivities of 10ohm-cm. or higher.

In accordance with this invention semi-insulating epitaxial deposits areformed by doping epitaxially formed Group III-V compounds with iron.Iron is incorporated into the epitaxial deposit during the growth of theepitaxial material from a source of iron arsenide. The iron arsenide isformed within the epitaxial reactor prior to the deposition of the GroupIII-V compound by a prearseniding step described hereinafter.

A particular advantage of this invention is the production of alternatelayers of semi-insulating and low resistivity semiconducting material inthe same monocrystalline wafer, thus providing electrical isolationbetween 9 Claims alternate layers of semiconducting material in amonocrystalline block. Other objects, features and advantages of theinvention Will become more readily understood from the followingdetailed description taken in conjunction with the appended claims andattached drawing, in which:

FIGURE 1 is an elevational view partially in section of an epitaxialreactor suitable for practicing the invention and FIGURE 2 is aperspective view in section of a monocrystalline water containingalternate layers of semiinsulating and semiconducting Group III-Vcompound materials.

The epitaxial reactor shown in FIGURE 1 comprises an elongatedcylindrical chamber 10 having end caps 11 and 12 fitted thereon. End cap11 carries feed tubes 13 and 14 passing therethrough into the interiorof chamber A boat-like container 15 is attached to the end of feed tube13, such that feed gases passing through feed tube 13 into the interiorof the chamber 10 pass over or through an iron source 16 contained inthe container 15. The container has an opening 17 to allow gases passingover or through the material 16 within the container to exit into theinterior of chamber 10. Likewise, a similar boat-like container 18attached to the end of feed tube 14 contains a Group III feed material19. The Group III feed material may be either elemental Group IIIelement or a compound thereof or alloys of a plurality of Group IIIelements. Container 18 is also attached in a manner so as to allow gasesentering through feed tube 14 to pass over or through the feed material19 before exiting into the interior of chamber 10 through opening 20.Substrate wafers 21 are positioned within the chamber 10 on a substrateholder 22 mounted within end cap 12. End cap 12 is also provided with anexhaust outlet 23 through which spent gases exit from the reactorchamber 10.

The reactor chamber 10 is positioned partially within a furnace havingthree separately controlled temperature zones, generally indicated bythe reference characters 1, 2 and 3. The heating elements of each of thezones of the furnace are individually controlled by conventional means(not shown) so that the temperature in each zone may be individually andseparately controlled to provide the desired temperature within eachzone of the furnace. The boat-like container 15 containing the ironsource 16, the feed container 18, and the substrate wafers 21 areappropriately positioned within zones 1, 2 and 3 of the reactor,respectively. Consequently, the temperature of the iron feed materialwithin the reactor chamber 10 is controlled by the temperature offurnace zone 1. Likewise the Group III feed material 20 is maintained atthe temperature of furnace zone 2, and the temperature of the substrates21 is determined by the temperature of furnace zone 3.

The operation of the apparatus of FIGURE 1 to produce epitaxial depositsof semi-insulating Group III-V compounds is discussed hereinafter withspecific reference to the production of semi-insulating galliumarsenide. However, it is to be understood that the description given ismerely by way of example, and that the principles of operation areequally applicable to other Group III-V compounds and ternary andquaternary alloys thereof.

With the exception of the iron doping step, as hereinafter described,operation of the apparatus of FIGURE 1 is similar to the operation ofconventional epitaxial reactors. Epitaxial deposits of gallium arsenideare conventionally formed by vapor phase reaction of gallium and arsenicin a thermal gradient to produce low resistivity gallium arsenidedeposits on the surface of the substrate. Zones 2 and 3 of the furnacein the reactor shown in FIGURE 1 and the apparatus contained therein aretypical of the conventional epitaxial reactor. An arseniccontaining gassuch as arsine (AsH or arsenic trichloride (AsCl is entrained in acarrier gas, usually hydrogen. The arsenic-containing gas is fed intothe reaction tube through inlet 14 and allowed to pass over feedmaterial 19, either gallium or gallium arsenide, contained in theboat-like container 18. The container 18 is maintained at a temperatureof about 8001100 C. Halide vapor produced by the reduction of AsCl orsupplied separately reacts with the gallium source 19 and forms avolatile chloride of gallium which is then transported from thecontainer 18 through exit 20 and into the reaction chamber.

The Group V element such as arsenic may be admitted through tube 14 inthe form of vapors of arsenic trichloride or any other suitable volatilecompound of arsenic. Alternatively, arsenic may be admitted into thereactor through a separate inlet tube (not shown) in the form of asuitable volatile hydride or halide of arsenic or in the form ofelemental vaporized arsenic transported in a carrier stream such ashydrogen. The resultant gaseous mixture contains elemental arsenic and avolatile halide of gallium which then flows through a decreasingtemperature gradient into zone 3 of the furnace. The volatile galliumhalide disproportionates into gallium trihalide and free gallium. Thefree gallium reacts with the free arsenic to form a monocrystallineepitaxial layer of gallium arsenide on the substrates 21.

In accordance with a specific embodiment of this invention, theepitaxial gallium arsenide deposit formed as described above is dopedwith iron. High purity iron 16 is placed in container within zone 1 ofthe furnace of FIGURE 1. Purified gallium 19 is placed in container 18and suitable substrates 21 such as monocrystalline gallium arsenidewafers are positioned on the substrate holder 22 within the reactor 10.The reactor is closed and flushed with hydrogen to remove oxygen andwater vapors from the reactor chamber. Individual controls are activatedto raise the temperatures of the furnace zones 1, 2 and 3 to the desiredtemperatures for the production of gallium arsenide. The temperature ofzone 1 is raised to about 900 C. to 950 C. Zone 2 is maintained at about800 C. to 1100 C. and zone 3 is maintained at about 500 C. to 800 C. Anarsenic-containing gas such as arsine, arsenic trichloride or elementalarsenic vapors entrained in a suitable carrier such as hydrogen isadmitted through inlet tube 13 and passed over the purified iron 16 incontainer 15. The arsenic reacts with the iron to form an arsenic-ironcompound (FeAs or FeAs This step is referred to as pre-arseniding theiron.

It has been found that iron will not be satisfactorily included in theepitaxial deposit unless pre-arsenided. Presumably, elemental iron isnot transported in sufiicient quantities to dope the epitaxial deposit;but iron reacts with arsenic to form a volatile iron arsenide compoundwhich is transported to the deposition zone by a suitable carrier gasand is included in the deposit, thereby doping the epitaxial depositwith iron. The amount of iron included in the epitaxial deposit, andthus the resistivity of the epitaxial deposit, is controlled bycontrolling the relative flow rates of carrier gases used fortransporting the gallium, arsenic and iron arsenide. The iron arsenideis preferably formed in the reactor immediately prior to use asdescribed above in order to insure purity of the iron source and avoidcontamination. However, other sources of iron arsenide may besubstituted. Furthermore, the iron arsenide is conveniently transportedin a carrier gas containing arsenic, thus replacing iron arsenideremoved by the carrier gas.

EXAMPLE I About 30 grams of elemental gallium was placed in container 18of the reactor shown in FIGURE 1. About 200 mgs. of purified iron wasplaced in container 15. The reactor was closed and flushed withhydrogen. With hydrogen in the reactor, furnace zones 1, 2 and 3 wereactivated so as to produce temperatures of 950 C., 950 C. and 750 C. inthe iron, feed and substrate zones respectively. A gaseous mixture ofabout 2% AsCl in hydrogen was admitted through feed tube 13 at a rate ofabout 50 cc./min. These conditions were maintained for 3 hours, duringwhich time the iron in boat 15 was converted to iron arsenide.

After the iron had been converted, the reactor was cooled, flushed withhelium and a monocrystalline wafer of GaAs 21 placed on substrate holder22. The reactor was closed and flushed with hydrogen. The furnace zoneswere activated to produce the same temperatures as before. A gaseousmixture of about 2% AsCl in H was passed through feed tube 14 at a rateof about cc./min. The flow of AsCl in H through feed tube 13 was reducedto 12 cc./min. Under these conditions an irondoped epitaxial layer ofGaAs was deposited on substrate 21 at a rate of about 0.3 microns/min.The epitaxial layer was examined and found to be P-type with aresistivity of 10 ohm-cm. at room temperature. The epitaxial layer wasmonocrystalline and uniform in composition and conductivity.

The wafer of FIGURE 2 is illustrative of the composite structures whichcan be made utilizing the invention. The wafer is comprised of aplurality of monocrystalline layers 31 and 32 epitaxially deposited on asubstrate 21. The substrate 21 may be a monocrystalline III-V compoundsemiconductor wafer or ternary or quaternary alloy. The substrate mayaso be germanium or silicon or any of the II-VI compounds withcrystalline latex spacings closely approximate to that of the epitaxialmaterial to be formed. Furthermore, the substrate 21 may be N-type,P-type on semi-insulating material. As shown in FIGURE 2, a plurality oflayers 31 and 32 may be epitaxially formed on the substrate 21. Forexample, substrate 21 may be low resistivity N-type GaAs. Epitaxiallayer 31 may be formed thereon as described above in Example I toproduce an epitaxial layer 31 of semi-insulating GaAs. A second layer 32may be epitaxially deposited on the first epitaxial layer 31 asdescribed above but eliminating the iron, thus forming a monocrystallinewafer having two layers of low resistivity GaAs contiguous with butelectrically separated by a layer 31 of semi-insulating GaAs. Devicesformed in GaAs of the low resistivity layers 21 and 32 are electricallyseparated by the semiinsulating layer 31. It will be recognized thatvarious other combinations of low resistivity and high resistivityepitaxial deposits can be used to fabricate various networks Within asingle wafer of monocrystalline material.

Although the invention has been described with specific reference toforming epitaxial deposits of semi-insulating gallium arsenide, it willbe noted that ternary and quaternary alloys of other III-V compounds canalso be formed in accordance with the invention through slightmodifications of the apparatus and deposition procedure.

EXAMPLE II A procedure similar to that of Example I was used to depositiron-doped epitaxial Ga(As, P). After the iron was converted asdescribed in Example I, the reactor was cooled, flushed with helium anda monocrystalline wafer of GaAs 21 placed on substrate holder 22. Thereactor was closed and flushed with hydrogen. The furnaces wereactivated to produce temperatures of 950 C., 950 C. and 775 C. in theiron, feed and substrate zones respectively. A gaseous mixture of about1% PC11; and 1% AsCl in hydrogen was passed through feed tube 14 at arate of about cc./min. The fiow of AsCl in H through feed tube 13 wasabout 30 cc./min. Under these conditions, an iron-doped epitaxial layerof GaAs P was deposited on substrate 21 at a rate of about 0.04microns/min. The epitaxial layer was examined and found to be P-typewith a resistivity of 2X10 ohm-cm. at room temperature. The epitaxiallayer was monocrystalline and uniform in composition and conductivity.

Monocrystalline epitaxial layers of Ga(As, P) ranging from GaAso ggpo gzto GaAs P have deposited on various semiconductor substrates by theprocess described in Example II. All deposits were doped with iron usingthe iron arsenide as described and were found to have resistivities ofohm-cm. or higher at room temperature. The layers having the higherphosphorous content were found to have higher resistivities.

Although the invention has been described with specific reference toforming semi-insulating layers of GaAs and Ga(As, P), it will beunderstood that epitaxial layers of other Group III-V compounds such asthe arsenides, phosphicles and antimonides of aluminum, gallium andindium as well as ternary and quaternary compounds and alloys of thesame may also be doped to high resistivity with iron in accordance withthis invention. It will be noted that since iron is a deep acceptor inGroup III-V compounds, higher resistivities are attained with highbandgap materials than with low bandgap materials.

It is to be understood that the above-described embodiments of theinvention are merely illustrative of the principles of the invention.The apparatus shown and described may be modified to introduce aplurality of volatile Group V elements or compounds to produce epitaxialternary and quaternary III-V deposits. Numerous other arrangements andmodifications may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

What is claimed is:

1. The method of doping epitaxially formed deposits of Group III-Vmaterials with iron wherein said Group III-V materials are formed byreacting a gaseous mixture comprising hydrogen, at least one Group Velement or volatile compound of a Group V element, and at least oneGroup III element or volatile compound of a Group III element in areaction chamber, comprising the step of introducing iron arsenidevapors into said reaction chamber during the formation of said depositsof Group III-V materials.

2. The method of claim 1 wherein said iron arsenide is introduced intosaid reaction chamber in a gaseous mixture produced by passing a carriergas over iron arsenide maintained at a temperature between about 900 C.and 950 C.

3. The method of claim 2 wherein said carrier gas is a gaseous mixturecomprising hydrogen and arsenic trichloride.

4. The method of making epitaxial deposits of high resistivity GroupIIIV compound materials on a substrate comprising the steps of:

(a) reacting a gaseous mixture of hydrogen and at least one Group Velement or volatile Group V compound with a gaseous mixture of hydrogenand at least one Group III halide within a reaction chamber, and

(b) introducing iron arsenide vapors into said reaction chamber wherebya Group III-V compound doped with iron is deposited.

5. The method of claim 4 wherein said high resistivity Group III-Vcompound is gallium arsenide, said at least one Group V element orvolatile Group V compound is arsenic or a volatile arsenic compound, andsaid at least one Group III halide is a gallium halide.

6. The method of claim 4, wherein said high resistivity Group III-Vcompound is Ga(As, P), said at least one Group V element or volatileGroup V compound is a mixture of PC1 and AsCl and said at least oneGroup III halide is a gallium halide.

7. In the process of making epitaxial deposits of Group III-V materialson a substrate wherein a gaseous mixture comprising hydrogen and atleast one Group V element or volatile Group V compound is mixed with agaseous mixture comprising hydrogen and at least one Group III halide ina reaction chamber to produce a monocrystalline deposit of Group III-Vmaterial, the method of doping said monocrystalline deposit with ironcomprising the steps of:

(a) reactng iron with arsenic to form iron arsenide,

and

(b) forming iron arsenide vapors and introducing said iron arsenidevapors into said reaction chamber during the formation of saidmonocrystaline deposit.

8. The method of claim 7, wherein said iron arsenide is introduced intosaid reaction chamber in a gaseous stream produced by passing a mixturecomprising hydrogen and arsenic trichloride over iron arsenidemaintained at a temperature between about 900 C. and 950 C.

9. The method of making epitaxial deposits of high resistivity galliumarsenide on a substrate comprising the steps of:

(a) passing a gaseous mixture comprising AsCl and hydrogen over ironmaintained at a temperature of about 950 C. in a reaction vessel therebyproducing iron arsenide,

(b) passing a gaseous mixture comprising AsCl and hydrogen over a sourceof gallium maintained at a temperature of about 950 C., therebyproducing a first reactant mixture comprising hydrogen, arsenic halides,arsenic and gallium halides,

(c) passing a gaseous mixture of hydrogen and AsCl over said ironarsenide maintained at a temperature of about 950 C., thereby producinga second reactant mixture comprising hydrogen, arsenic halides, arsenicand iron arsenide, and

(d) mixing said first reactant mixture with said second reactant mixturein a decreasing temperature gradient.

References Cited UNITED STATES PATENTS 2,778,802 1/ 1957 Willardson24262.3 3,146,137 8/1964 Williams 148-175 3,218,205 11/1965 Ruehrwein148--175 3,226,225 12/1965 Weiss et al 25262.3 XR 3,277,006 10/1966Johnson et a1. 148175 XR 3,310,425 3/1967 Goldsmith 148-175 XR 3,312,5704/1967 Ruehrwein 148175 3,344,071 9/1967 Cronin 25262.3 3,364,084 1/1968 Ruehrwein 148-175 L. DEWAYNE RUTLEDGE, Primary Examiner. PAULWEINSTEIN, Assistant Examiner.

US. Cl. X.R.

1. THE METHOD OF DOPING EPITAXIALLY FORMED DEPOSITS OF GROUP III-VMATERIALS WITH IRON WHEREIN SAID GROUP III-V MATERIALS ARE FORMED BYREACTING A GASEOUS MIXTURE COMPRISING HYDROGEN, AT LEAST ONE GROUP VELEMENT OR VOLATILE COMPOUND OF A GROUP V ELEMENT, AND AT LEAST ONEGROUP III ELEMENT OR VOLATILE COMPOUND OF A GROUP III ELEMENT IN AREACTION CHAMBER, COMPRISING THE STEP OF INTRODUCING IRON ARSENIDEVAPORS INTO SAID REACTION CHAMBER DURING THE FORMATION OF SAID DEPOSITSOF GROUP III-V MATERIALS.